feat(server+web): 客户端下载闭环——pangolin-server /downloads + 官网下载链接

- server: /downloads/* 公开静态服务(NewDownloadsHandler,DOWNLOADS_DIR 默认
  /var/lib/pangolin/downloads),防目录穿越/不列目录/缺目录不崩,Cache-Control:
  no-cache 保证 CI 覆盖后永远最新。挂在 /healthz 同级(免鉴权)。含 4 项测试。
- deploy.sh: install -d 下载目录 + server.env 注入 DOWNLOADS_DIR。
- web: site.ts 增 downloads.{android,windows}(→ https://api.yanmeiai.com/downloads/*);
  Download.astro 接上 Android/Windows 按钮,iOS/macOS/Linux 改「即将推出」禁用态。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

server/cmd/server/main.go

@@ -130,6 +130,12 @@ func main() {
 		_ = json.NewEncoder(w).Encode(map[string]string{"status": "ok"})
 	})
 
+	// Public (no auth): 客户端安装包下载(官网下载按钮直链)。CI
+	// (scripts/ci/deploy-client.sh)把最新安装包 scp 到 DOWNLOADS_DIR,按平台固定
+	// 文件名覆盖;目录不存在也不影响启动,只是请求 404(见 DownloadsHandler 注释)。
+	downloadsHandler := httpapi.NewDownloadsHandler(os.Getenv("DOWNLOADS_DIR"))
+	r.Get("/downloads/*", downloadsHandler.Serve)
+
 	// Optional probe ingest route. sharedProbeStore is reused by the scheduler
 	// (below) when both are enabled, so they share one Redis-backed store.
 	var sharedProbeStore *probe.Store